In the related art, known is an electric motor in which a stator disposed in a region surrounding a rotor is configured by split stators split in a circumferential direction (for example, see JP 2002-95192A (Reference 1) and JP 2015-65741A (Reference 2)). In this manner, if the stator is configured by the split stators, a coil is likely to be wound around the stator in an aligned manner, densification of winding is achieved, and the electric motor becomes compact.
A split stator of Reference 1 includes a split yoke portion, and a teeth portion that protrudes toward an inside in a radial direction from the split yoke portion, in which a projection portion and a recess portion are formed on end surfaces of the adjacent split yoke portions. In a state where the projection portion and the recess portion are fit, an outer circumferential side of a fitting portion of the adjacent split yoke portions is bonded by a laser welding. Reference 1 discloses that shapes of bonding surfaces between the recess portion and the projection portion are different from each other on an outside and an inside, thereby, reducing clattering and reducing a deformation amount due to thermal contraction.
A split stator of Reference 2 includes a split yoke portion, and a teeth portion that protrudes toward the inside in the radial direction from the split yoke portion, in which a projection portion and a recess portion are formed on the end surfaces of the adjacent split yoke portions. In a state where the projection portion and the recess portion are fit, and each of the split stators is pressed on the inside in the radial direction by using a ring-shaped jig which is capable of expanding and contracting in the radial direction, the split stator is electrified, and diffusion bonding is performed.
However, since the electric motor of the related art is formed by bonding by the laser welding or the diffusion bonding in a state where the projection portion and the recess portion are fit on the end surfaces of the adjacent split yoke portions, a distortion due to thermal stress easily becomes ununiform between the projection portion and the recess portion. As a result, a circularity of an inner circumferential surface of the stator is lowered, an interval between the stator and the rotor becomes ununiform in the circumferential direction, and degradation of motor performance is caused. In particular, as Reference 1, in a case where the outer circumferential side of the bonding portion of the split yoke portions is bonded by the laser welding, the circularity of the inner circumferential surface of the stator is likely to be lowered since heating is locally performed.
In a case where the diffusion bonding is performed as Reference 2, since the entire stator is heated, power consumption becomes large, and there is a need to adjust a temperature condition or electrification time in order to bond the split yoke portions, and it is complicated. Furthermore, an electrical current may concentrate on the projection portion, and the distortion due to the thermal stress may become too large.
Thus, a need exists for an electric motor which is not susceptible to the drawback mentioned above.